Fluidized bed combustion furnaces need a free board in order to resettle a fluidizing medium, for example, sand, scattered at the fluidized bed section. If the flow velocity of the combustion gas ascending through the free board section is excessively high, the fluidizing medium scatters outside the free board section; therefore, the flow velocity of the combustion gas at the free board section is restricted to about 2 m/s. Accordingly, the free board section is generally arranged such that the cross-sectional area (horizontal section area) of the free board section is larger than that of the fluidized bed section.
In this arrangement, however, since the flow velocity of the combustion gas at the free board section is low, even if secondary air is supplied to the free board section, it is difficult to effect proper mixing of the unburnt gas and air, resulting in a lowering of the secondary combustion efficiency. In order to promote the mixing of such unburnt gas and air, various proposals have been made regarding the method by which air is supplied to the free board section. However, due to the wide cross-sectional area of the free board section the advantageous effects of these proposals are not able to be satisfactorily realized in the present state of the art.
There has also been proposed an arrangement wherein a throttle section is provided above the fluidized bed, as disclosed, for example, in Japanese Utility Model Public Disclosure (KOKAI) No. 62-18510. In this proposed arrangement, however, if the degree of throttling effected by the throttle section is excessively high, the flow velocity of the fluidizing medium becomes higher than the "terminal velocity" of grains or particles having a mean diameter and consequently a large amount of fluidizing medium is scattered outside the free board section, which necessitates incorporation of a means for returning scattered sand.
There is another problem that a large number of dead spaces which do not contribute to combustion are produced in the space extending between each pair of adjacent throttle portions which are provided in a multistage structure and in the space created due to the configuration of the free board section and therefore air which is blown into the furnace as secondary air, for example, cannot be effectively utilized.
Accordingly, to overcome the problems in the above-described equipment it is necessary either to reduce the degree of throttling effected by the throttle section or to provide a means for returning entrained sand.
To raise the temperature of a fluidizing medium, for example, sand, in conventional fluidized bed combustion furnaces, it is common to use an auxiliary burner or to lower the excess air ratio by increasing the burning rate. However, employment of an auxiliary burner necessitates the use of an auxiliary fuel, which is uneconomical, and an operation utilizing a low excess air ratio is problematic in that it generates such unburnt gases as CO and NH.sub.3.
In view of the above-described circumstances, it is an object of the present invention to provide a fluidized bed combustion furnace wherein the flow velocity of combustion gas in the throttle section is made higher than the terminal velocity (about 2 to 8 m/s) of mean diameter grains or particles of a fluidizing medium constituting the fluidized bed, and is effectively settling the scattered fluidizing medium in the free board section, thereby minimizing scattering of the fluidizing medium outside the free board section and improving the mixing of unburnt gas and secondary air, and wherein high-temperature combustion gases which are separated off from the free board section are caused to collide with each other in a junction chamber, thereby completely burning any trace amount of unburnt matter in the junction chamber, without the need for an auxiliary fuel to raise the temperature of the fluidizing medium constituting the fluidized bed and without any generation of unburnt gases such as CO and NH.sub.3.